1. Field of the Invention
The present invention relates to a control apparatus of resistance welders and, more particularly, to a control apparatus of resistance welders, which can improve voltage control precision of single-phase full-wave resistance welders using a thyristor.
2. Description of the Related Art
Resistance welders are also called a "spot welder", and are widely used in general industrial applications such as an assembly line of a vehicle. Of the resistance welders, a single-phase full-wave resistance welder is of most popular type.
The single-phase full-wave resistance welder controls a primary voltage applied to a welding transformer using a switching element such as a thyristor, thereby controlling a secondary current of the welding transformer, i.e., a welding current.
As a control system, a current control system wherein a primary or secondary current of the welding transformer is fed back and a voltage control system wherein a voltage applied to a primary side of the welding transformer is fed back are known.
The current control system allows high-precision welding current control but requires a current detector such as a current transformer, search coil, and the like, resulting in an expensive system. In this case, when the welding transformer has two or more guns, if there is a gun through which no welding current flows, a current flowing through the remaining gun is increased accordingly. Meanwhile, the voltage control system cannot achieve high-precision welding current control but is inexpensive and economical.
FIG. 2A shows a waveform when a power supply voltage is represented by v, a primary current of the welding transformer is represented by i, and firing angle .theta.n is 90.degree. in the single-phase full-wave resistance welder.
Since the welding transformer has a power factor&lt;1, current i is monotonously increased from zero to .theta.n, and is then monotonously decreased and returns to zero at .theta.m, thus completing half-cycle energization. In the next half cycle, a similar current flows in an opposite polarity. A power supply voltage indicated by hatching in the range of .theta.n to .theta.m is applied to the welding transformer in a half-cycle energization interval.
In the voltage control system, the voltage of the hatched portion is detected and fed back every half cycle.
When the voltage of the hatched portion is detected at every given sampling time to obtain an effective value, since initial and end values of this voltage are not zero, a detection error is generated depending on sampling start and end timings. If a sampling period is shortened, the detection error can be eliminated. However, if another control, monitoring, and the like are performed together, processing capacity of a microcomputer of a control apparatus is exceeded. As a result, a plurality of microcomputers are necessary, resulting in an expensive system.